DESCRIPTION (Adapted from the Abstract): Pseudomonas aeruginosa is a ubiquitous organism which maintains the ability to adapt to almost any environmental situation. This allows the organism to exist very effectively in the absence of a eukaryotic host or an infectious process. If, however, P. aeruginosa is allowed entry into a host due to damage of normal physical or immunological barriers, it produces infections with high morbidity and mortality. Both the adaptability and pathogenicity of P. aeruginosa are multifactorial and reflect the organism s ability to produce a variety of cell associated and extracellular products. The focus of this proposal is one of these products, an ADP-ribosylating enzyme, exoenzyme S (ExoS). ExoS production has been associated with increased P.aeruginosa virulence, but the function of ExoS in vivo remains unknown. ExoS production, however, is not unique to clinical isolates, with >90% of the P. aeruginosa environmental isolates also being found to produce ExoS. This implicates the importance of ExoS to the survival of the organism in the environment. The hypothesis of the proposed studies is that an increased understanding of the regulation and role of ExoS in the environment will help elucidate the role of ExoS in the infectious process. In the first specific aim of this project ExoS produced by soil and clinical P. aeruginosa isolates will be compared to determine if biochemical, enzymatic or nucleotide sequence differences exist between the two sources of ExoS. In the second specific aim, soil conditions associated with enhanced ExoS production will be defined to gain an increased understanding of mechanisms of ExoS regulation. In the third specific aim, the role of ExoS in the survival of P. aeruginosa in the soil will be examined to help clarify the function of ExoS in vivo. An understanding of the expression of ExoS in nature should result in new insights into the evolutionary conservation of highly regulated an metabolically expensive protein and will help to explain the role of ExoS in the production of disease in clinical situations.